The invention concerns a control device for an actuator.
The invention is described in keeping with a pneumatic double-acting cylinder used as actuator, but of course it is understood that the invention can be implemented whatever the type of actuator as, for example, a hydraulic cylinder whether single or double-acting.
To control a pneumatic double-acting cylinder by means of a valve usually referred to as 4-3 comprising four-way three position control is known in the art. A set-point, for example electric, is converted into compressed air which is used to move the air slide valves, for example by using a conversion device known as vane nozzle. In such a device, the electric set-point passes through an electromagnet designed to move a vane plugging or not a nozzle supplied by a source of compressed air, in keeping with the set-point instruction status.
In industry, a level of electric set-point giving out a current of between 4 and 20 milliamps with a voltage in the region of 10 to 30 volts is used currently. With a vane-nozzle device, the electric energy of the set-point is directly converted into compressed air without it being necessary to provide supplementary energy to carry out the conversion. The vane-nozzle devices are not very accurate and consume a considerable quantity of compressed air due to a practically continuous air leak from the nozzle.
To alleviate the disadvantages of vane-nozzle devices, the conversion has been carried out using monostable solenoid valves. These solenoid valves are far more accurate than the vane-nozzle devices, but the energy provided by the electric set-point is not sufficient to cycle the solenoid valves. Conventional solenoid valves require electric power in the region of one Watt in order to be cycled. To do this, means supplied by a source of outside electric voltage for amplifying the set-point are used, for example 24 Volts.
Because of this outside voltage source, one cannot directly replace a vane-nozzle device by conversion means comprising monostable solenoid valves. Moreover, supply by the outside voltage source prevents obtaining a homologation in intrinsic safety. More precisely, it is possible to use a system accredited in inherent safety in an explosive atmosphere without an explosive-proof jacket being necessaryxe2x80x94what is more the outside source of voltage requires such an explosive-proof casing.
Recent developments in the field of solenoid valves facilitate avoiding recourse to an outside voltage source by using piezoelectric solenoid valves which can be controlled directly by a set-point of between 4 and 20 milliamps. On the other hand, piezoelectric solenoid valves present a number of disadvantages, notably their much higher price compared to conventional monostable solenoid valves and their low flux density displacement, in this way restricting the air flow which they are likely to switch over, and consequently their ability to move an air slide valve.
The aim of the invention is to alleviate these various disadvantages by offering a control device for an actuator which is far more accurate and which consumes less air than a vane-nozzle device, using neither outside voltage source nor piezoelectric solenoid valve.
To attain this aim, the invention has an actuator control device controlled by a control valve comprising the means to obtain different valve conditions, typified in that the means to obtain different valve conditions are activated by at least one two-position fluidics solenoid valve to which a set-point is applied and in that the control device comprises the means to be assured of the response of the solenoid valve depending on the set-point over the whole range that the actuator can cover.
The means to obtain different conditions of the control valve usually comprise slides. It is possible to replace, for example, these slides by rotary valves, cylinders or check valves likely to block nozzles. To simplify further description, one will use the term xe2x80x9cslidexe2x80x9d to describe means to obtain different conditions of the valve whatever the method of realization of these means.